The present invention relates to polymeric compositions having both siloxane units and imide units. More particularly, the present invention relates to siloxane-imide copolymers which are crystalline in nature.
It is known in the art that siloxane-imide copolymers can be prepared by reacting, for example, a diaminosiloxane and a dianhydride such as benzophenone dianhydride to yield a polyamide acid having the formula ##STR1## where R is a divalent hydrocarbon radical, R' is a monovalent hydrocarbon radical, m is a whole number greater than one, and n is a whole number greater than 10. Such polyamide acids are soluble in highly polar solvents such as N-methyl pyrrolidone and are provided the end-user in this form.
The siloxane-amide polymer is formed typically after the end-user has applied a coating of polyamide acid to a substrate, by heating at a temperature of from about 150.degree. C. to about 400.degree. C. to remove the solvent and effect cyclization to form a siloxane-imide copolymer having, for example, the formula ##STR2##
These polyimides, while useful as protective coatings for semiconductors and other electronic devices, suffer from the defect that they are insoluble in most low boiling organic solvents. Another drawback of methods which require heating the polyamide acid at temperatures of from 150.degree. C. to 400.degree. C. is that many semiconductor devices cannot be heated to such extremes without adversely affecting the device itself. Furthermore, the artisan will appreciate that the amide acid can hydrolyze to form carboxylic acid groups which, of course, will prevent complete imidization when the end-user attempts to use the product. Accordingly, it is desirable to provide a polyimide capable of being applied in the form of an imide rather than in the form of an amide acid.
Holub, U.S. Pat. No. 3,325,450, discloses polyimidesiloxanes of Formula II hereinabove and their preparation by reacting diaminosiloxanes and organic dianhydrides to form a polyamide acid, and thereafter heating the polyamide acid to effect imidization. Variations of Holub's teachings can be found in U.S. Pat. Nos. 3,392,144; 3,435,002; 3,553,282; 3,558,741; 3,663,728; and 3,740,305.
Berger, U.S. Pat. No. 4,030,948, discloses a polyimide copolymer which is the reaction product of a tetracarboxylic acid dianhydride, an organic diamine and a di(aminoalkyl)polysiloxane, where the di(aminoalkyl)polysiloxane constitutes 18 to 45 mole percent of the total amine requirement of the polymer.
Berger, U.S. Pat. No. 4,395,527, discloses that polyimides containing siloxane units of the formula ##STR3## where Q is a substituted or unsubstituted aromatic group; ##STR4## D is an unsubstituted or substituted hydrocarbylene; R.sup.1, R.sup.2, R.sup.3, R.sup.4, R.sup.5, and R.sup.6 are each, independently, unsubstituted or substituted hydrocarbyl radicals; and x, y and z each, independently, have a value from 0 to 100; impart improved solubility and adhesion to the polyimide. Generally, such polyimides are said to be soluble in chlorinated hydrocarbon solvents such as dichlorobenzene and trichlorobenzene, as well as in polar solvents such as N,N-dimethyl acetamide; N-methyl caprolactam; dimethylsulfoxide; N-methyl-2-pyrrolidone; tetramethylurea; pyridine; dimethylsulfone; hexamethylphosphoramide; tetramethylene sulfone; formamide; N-methylforamide; butyrolactone; and N-acetyl-2-pyrrolidone. Berger further teaches that if a diether-containing anhydride is utilized as one of the starting materials, there is obtained a polyimide soluble not only in the chlorinated hydrocarbon solvents and polar solvents previously disclosed, but also, where it contains a siloxane unit, the polyimide is soluble in a solvent which is derived from monoalkyl and/or dialkyl ethers of ethylene glycol and condensed polyethylene glycols and/or cyclic ethers containing no less than a five member ring, such as diglyme. However, Berger makes clear that polyimides will have limited solubility in diglyme. The artisan will appreciate that Berger requires the use of unusual monomers which, accordingly, are rather expensive if it is desired to obtain a diglyme soluble imide-siloxane polymer. In another aspect Berger teaches the art that polyimides containing siloxane units have a much lower glass transition temperature (Tg), e.g. on the order of 140.degree. C. as compared with 350.degree. C. for conventional polyimides. Consequently, they will melt and flow more readily than prior art polyimides.